1. Field of the Invention
The present invention relates to a three-grating type photoelectric encoder which is provided with a second grating formed on a scale and first and third gratings disposed on the side of a detector. In particular, the present invention relates to a photoelectric encoder which can effectively remove harmonic components.
2. Description of the Related Art
In a so-called three-grating principle used in a linear encoder, as described in, for example, Patent Document 1 (Japanese Patent Laid-Open Publication No. Sho 63-33604, see FIGS. 1 to 3 thereof), three optical gratings serve as a spatial filter. For a transmission type shown in FIG. 1(A), the three optical gratings are a second grating 20 serving as a main scale and first and third gratings 22 and 24 serving as an index scale. For reflection types shown in FIGS. 1(B) and 1(C), light passes through a common first grating 22 (also serving as a third grating) twice. Therefore, it is possible to detect an approximately sinusoidal signal and carry out interpolation with high precision. In FIGS. 1(A) to 1(C), reference numeral 26 represents a light source, and reference numeral 28 represents an optical system composed of, for example, a collimator lens. Reference numeral 30 represents a light receiving element, and reference numeral 32 represents a half mirror employed in one reflection type. Reference numeral 34 represents a mirror employed in another reflection type, and reference numeral 36 represents a condensing lens employed in the reflection type.
When the three-grating principle is adopted in the reflective encoder, as shown in FIGS. 1(B) and 1(C), a scale grating serves as a second grating 20. In the optical grating on a scale, however, line width varies in accordance with the position of the scale. Therefore, there is a problem that a harmonic component is superimposed on an output signal at that portion.
Accordingly, Patent Document 2 (Japanese Patent Laid-Open Publication No. Hei 9-196706, see FIG. 5) and Patent Document 3 (Japanese Patent Laid-Open Publication No. 2004-264295, see FIG. 4), as shown in FIG. 2, have proposed methods in which third gratings 24 in front of a light receiving element 30 and the light receiving element compose a light receiving element array 31 to stabilize an output signal by averaging effect of the light receiving array.
Furthermore, in the structure of FIG. 2, methods for providing phase difference (Patent Document 4: Japanese Patent Laid-Open Publication No. Hei 10-122909, see FIG. 2) or line width modulation (Patent Document 5: Japanese Patent Laid-Open Publication No. Hei 8-145724, see FIG. 2) have been proposed to remove a harmonic component.
Patent Document 6 (Japanese Patent Laid-Open Publication No. Hei 9-113213, see FIG. 3) has proposed a method in which the grating patterns of a first grating and a third grating have a phase difference in a detection principle using the three-grating principle.
However, the methods in which the light receiving array has the phase difference or the line width modulation as described in Patent Documents 4 and 5 need to vary the pattern of the light receiving element, thereby increasing manufacturing costs.
In the method in which the grating patterns of the first grating and the third grating have a phase difference as described in Patent Document 6, however, if a light receiving array is used instead of the third grating, only harmonic components in the order of even numbers can be removed because light passes through the same grating twice. Therefore, it is impossible to remove harmonic components in the order of odd numbers, in particular, the harmonic component of the 3rd order which is larger than the harmonic of the 2nd order and becomes the biggest problem. Hence, there is a problem that the light receiving array cannot be used instead of the third grating.
The above problems reside not only in the reflection type encoder but also in the transmission type encoder.